According to conservative estimates, as a consequence of their contamination by chloride salts, as many as one-half of all highway bridges in the United States are deteriorating due to reinforcing bar (rebar) corrosion. A similar situation prevails in the United Kingdom. Rebar corrosion in bridges is an outcome of the repeated wintertime application of deicing salts such as sodium chloride and calcium chloride. Chloride-induced corrosion destroys the rebar and causes formation of corrosion products. The corrosion products occupy a greater physical volume than the rebar itself and cause internal expansion which leads to cracking and spalling of the concrete cover. Once cracking and spalling have occurred, the rebar is accessible to further chloride-induced corrosion--and so conditions deteriorate rapidly. The principal factors that must be controlled to inhibit corrosion include oxygen, chloride ion, water, and pH. Approaches to control these factors have used inhibitors, electrochemical protection procedures, scavengers, buffers, and coatings.
The use of corrosion inhibiting agents is a traditional approach to preventing or slowing the corrosion of steel. Most corrosion inhibitors for steel find use in acidic or neutral conditions--where uninhibited attack may be very rapid; by comparison, corrosion of steel under alkaline conditions experienced by rebar in concrete is very slow. Because a long life is required of reinforced bridges and since corrosion is induced by chloride contamination, the use of corrosion inhibitors in protecting rebar is a natural consideration. At least two corrosion inhibitors are currently in use for the protection of rebar. The use of calcium nitrite in concrete mix has become fairly widespread during the past decade, and concrete made with added calcium nitrite has considerable resistance to chloride-induced corrosion. A different approach to inhibition of chloride-induced rebar corrosion involves the use of a corrosion-inhibiting additive as a minor component of the road salt. To this end, sodium monofluorophosphate has been employed.